Post-test evaluation of oxygen electrodes from solid oxide electrolysis stacks☆
Introduction
High-temperature steam electrolysis (HTSE) is a high-efficiency process that generates hydrogen by water-splitting using electricity and heat from an advanced nuclear power plant [1], [2]. The process utilizes multiple ceramic cells arranged in “stacks”, which are identical to solid oxide fuel cell designs, however, for HTSE, the stacks are operated with a reversed current flow and use a mixture of steam and hydrogen as the feed gas. The “stacking” of cells provides for a reasonable electrolysis voltage and current, as well as a cell size that is easy to manufacture. However, both increased cell area and a multiplicity of components in the stack introduce many potential sources of defects that can lead to performance degradation over time.
A recent article from Idaho National Laboratory reports the results of a 1000-h test of a Ceramatec, 25-cell planar HTSE stack [2]. This test was conducted under constant applied voltage at 830 °C using a feed gas composition of 54% H2O, 37% N2 and 9% H2. The stack, which had generated hydrogen at a mean rate of 177 NL/h, experienced an overall performance degradation of ∼20% over the duration of the test [2]. In 2006, Ceramatec conducted an HTSE demonstration using an integrated laboratory scale (ILS) half-module, which consisted of two stacks of 60 planar solid oxide electrolysis cells, along with the necessary bipolar plates and flow fields assembled in a cross-flow configuration [3]. The test was conducted for 2055 h under constant applied voltage. The temperature of the stack was initially 800–825 °C and had risen to 845–865 °C near the end of the test. The initial feed gas composition was 85% H2O, 12% H2 and 3% N2. After 168 h, CO2 was fed in with the steam (H2O:CO2 = 1:1) to generate syngas (CO + H2) for 1000 h, then the feed gas was returned to the initial composition for the duration of the test. This stack produced 1250 NL/h of hydrogen initially, but experienced an overall performance degradation of ∼46% over the duration of the test, most of which occurred in the first 1000 h. This degradation did not appear to be affected by the introduction of or cessation of CO2 into the feed gas.
After each of these two tests, the stacks were disassembled and a few cells, along with bipolar plates and flow fields, were provided to Argonne for post-test analyses. The objective of this work was to identify the causes of performance degradation within the HTSE stacks. Our approach was to map the surface characteristics of cells and bipolar plates obtained from the disassembled stacks using four-point resistivity measurements and X-ray fluorescence (XRF), followed by closer examination of selected areas using X-ray absorption near edge structure (XANES) measurements, Raman micro-spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS). These techniques yielded information such as in-plane resistance, elemental distribution, oxidation state, phases present, electrode delamination and porosity within the electrode layers. In this study, we discuss the results of our investigation of the oxygen electrode compartment of the solid oxide electrolysis (SOEC) cells.
Section snippets
Experimental procedures
Cells and bipolar plates were separated from the 25-cell Ceramatec stack [2] and ½-ILS module [3] for analysis. The electrolyte-supported cells have a 200-μm-thick scandia-stabilized zirconia electrolyte, graded oxygen electrodes consisting of strontium-doped rare earth manganese perovskite oxide mixed with zirconia and (La,Sr)CoO3 bond layers. The steam/hydrogen electrodes consisted of a nickel-ceria cermet and a nickel bond layer. A schematic representation of a stack repeat unit is shown in
Four-point resistivity probe
The four-point probe is a simple and fast method to identify areas of degradation by mapping the resistivity of the surfaces. The maps in Fig. 2 illustrate the resistivities of a 1000-h cell and bipolar plate in the oxygen compartment. Fig. 2(a) shows the results for the oxygen electrode surface after the bond layer was removed. Maps of the bond layer and the electrode for this cell showed similar results. The resistivity is reasonably constant over most of the electrode surface; however, it
Discussion
From the results presented above, we concluded that there are two primary degradation phenomena affecting the oxygen electrodes in high-temperature steam electrolysis stacks. The first is the chromium contamination of the bond-coat layer and, to a lesser extent, the oxygen electrode layers beneath. The second and more damaging phenomenon is delamination of the oxygen electrode. The proposed mechanisms by which these two phenomena occur and their implications on stack performance are discussed
Conclusions
Two high-temperature steam electrolysis stacks that had operated for over 1000 and 2000 h, respectively, at ∼830 °C were disassembled and the oxygen electrodes in some of the cells were examined to identify potential causes of stack performance degradation over the test periods. Using the composition and conductivity mapping approaches with XRF and four-point resistivity measurements, we were able to identify regions of interest where performance degradation most likely occurred. We then used
Acknowledgements
The authors are grateful to Deborah Myers, Mark Petri, Theodore Krause, Terry Cruse, John Vaughey, Ann Call, Magali Ferrandon and Nathan Styx of Argonne National Laboratory for their valuable assistance and input. Thanks to Joseph Hartvigsen, S. Elangovan and Dennis Larsen of Ceramatec, Inc. for supplying the stack components for this study and for their useful input. Thanks to Steven Herring, Carl Stoots, Jim O'Brien and Grant Hawkes of Idaho National Laboratory for their support and helpful
References (30)
- et al.
Hydrogen production by high temperature electrolysis of water vapor
Int J Hydrogen Energy
(1980) - et al.
Differential redox and sorption of Cr(III/VI) on natural silicate and oxide minerals: EXAFS and XANES results
Geochim Cosmochim Acta
(1997) Effect of cathode and electrolyte transport properties on chromium poisoning in solid oxide fuel cells
Int J Hydrogen Energy
(2007)Thermodynamics of gas-phase chromium species—the chromium oxides, the chromium oxyhydroxides, and volatility calculations in waste incineration processes
Combust Flame
(1993)- et al.
Vaporisation of chromia in humid air
J Phys Chem Solids
(2005) - et al.
Standard molar Gibbs energies of formation of the ternary compounds in the La–Co–O system using solid oxide galvanic cell method
J Alloys Compd
(1999) - et al.
Oxide ion transport in undoped and Cr-doped LaCoO3−δ
Solid State Ionics
(2007) - et al.
The geometry dependence of the polarization resistance of Sr-doped LaMnO3 microelectrodes on yttria-stabilized zirconia
Solid State Ionics
(2002) - et al.
Performance of planar high-temperature electrolysis stacks for hydrogen production from nuclear energy
Nucl Technol
(2007) - Hartvigsen JJ, Elangovan S, Nickens A. Test of high temperature electrolysis ILS half module. In: U.S. DOE Hydrogen...
Sheet resistivity measurements on rectangular surfaces-general solution for four point probe conversion factors
Bell Syst Tech J (USA)
Measurement of sheet resistivities with the four-point probe
Bell Syst Tech J (USA)
Quantitative X-ray spectrometry
Potassium-assisted chromium transport in solid oxide fuel cells
J Electrochem Soc
X-ray absorption: principles, applications, techniques of EXAFS, SEXAFS, and XANES
Cited by (190)
Theoretical understanding of stability of the oxygen electrode in a proton-conductor based solid oxide electrolysis cell
2023, International Journal of Hydrogen EnergyDesigning the nano-scale architecture of the air electrode for high-performance and robust reversible solid oxide cells
2023, Applied Catalysis B: EnvironmentalUtilizing oxygen gas storage in rechargeable oxygen ion batteries
2023, Journal of Power Sources
- ☆
The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.